In a factory where food and the like are produced, a product is automatically inspected during its transport by a conveyor or the like using an article inspection apparatus for checking the presence of a defect such as foreign object intrusion and a missing ingredient in the product, and the product is categorized as a normal product or a defective product.
Particularly, a product such as food needs to be closely inspected for the presence of an intruding foreign object such as metal or plastic. Therefore, an article inspection apparatus that uses an X-ray has been developed in recent years.
Generally, in the article inspection apparatus that uses an X-ray, an X-ray that has a width in a direction orthogonal to the passing direction of an inspected article is emitted to a passage of the inspected article. The X-ray that is transmitted through the inspected article is received by a plurality of sensor elements that are arranged in the direction orthogonal to the passing direction of the inspected article. Image information that represents a difference in X-ray transmittance between each part of the inspected article with light and shade is acquired, and various processes are performed on the image information to determine the presence of foreign object intrusion or the presence of a missing or damaged ingredient and the like.
In addition to a change in transmittance caused by the presence of foreign object intrusion, the presence of a damaged or missing ingredient, and the like in the inspected article, a difference in distance to each X-ray sensor element from the X-ray source due to the spreading emission of the X-ray toward an inspection region makes the intensity of the X-ray incident on each sensor element non-uniform in the article inspection apparatus that uses the X-ray.
In addition, a difference in sensitivity between each sensor elements occurs, and a difference in sensitivity between each array or each module also occurs in an arrangement of a plurality of arrays or modules, each of which is configured with a plurality of sensor elements.
For example, as illustrated in FIG. 13, in a state of the absence of the inspected article, if an X-ray that is generated by an X-ray generation source 1 is emitted from above the center in the width direction of a transport belt 2 and spreads in a fan shape in the width direction of the belt, and the X-ray that is transmitted through the transport belt 2 is detected by sensor elements 3a, 3a, . . . , 3a arranged in the width direction of the transport belt 2, a distance L to each of the sensor elements 3a, 3a, . . . , 3a from the X-ray generation source 1 is the shortest at the center and is increased toward both ends as illustrated in FIG. 14. Accordingly, as illustrated by a dotted line, the output of each of the sensor elements 3a, 3a, . . . , 3a (or the shade of an image acquired from the output) is theoretically the highest in the central part and tends to be decreased toward both ends as opposed to the change in the length L. In addition to such a change, a difference in sensitivity between each array, a difference in sensitivity between each sensor element in the array, and the like cause the output value of each sensor element to vary and also affect the image acquired from the output.
Accordingly, the output of each sensor element needs to be corrected in advance in order to correctly acquire the X-ray transmittance of each part of the inspected article when the article for inspection is transported.
As such a correction method, correction means for performing correction that equalizes the output of each sensor element or the shade of the image acquired from the output at a transmittance of 100% given that the transmittance in the state of the absence of the inspected article is set as 100% is provided in the related art.
For example, such a method of performing correction in the state of the transmittance of 100% is disclosed in Patent Document 1.